Giga-annum

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Giga-annum, abbreviated Ga, is a unit of time equal to one thousand million years. It is commonly used in scientific disciplines such as cosmology and geology to signify extremely long time periods in the past. For example, the formation of the Earth occurred approximately 4.55 Ga, that is, 4.55 thousand million years ago.

Within scientific literature, the abbreviation "Ga" replaces the notation "Gya".

"Giga" is Latin meaning "thousand million" or 109; and comes from the Greek "gigas" meaning "giant". It is also used with the same meaning as a prefix in the SI. "Annum" comes from the Latin word meaning "year".

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